Spectrophotometric Determination of Anionic Surfactant Based on Ion-pair Formation with Methylene Blue in Reversed Flow Injection Mode

A simple reversed flow injection system with minimal consumption of organic solvent has been developed for the spectrophotometric determination of anionic surfactant (AS) in water samples. The method is based on the ion-pair formation between anionic surfactant and Methylene Blue (MB) and its extraction to chloroform. Sodium dodecyl sulfate (SDS) is used as a model anionic surfactant. The sample and MB solutions are each delivered by two peristaltic pumps and merged to form the ion-pair of AS and MB. Small amounts of chloroform (50 mm) are injected every 15 s to the merged solution. The ion-pair is extracted to the chloroform phase while the solution is passing through an extraction coil. The absorbance of both phases (aqueous and organic phases) is measured sequentially at 652 nm without phase separation. Analytical parameters such as extraction coil length and MB concentration have been optimized. The dynamic ranges of calibration curves (0.1 ~ 1 and 2 ~ 8 μmol dm) are selected depending on the analyte concentration by changing MB concentration. Calibration curves are linear (r > 0.997) over the ranges; LOD (3.3 σ) is 0.054 μmol dm. Relative standard deviations of repeated measurements are 2.44% (n = 8, CSDS = 1 μmol dm) and 1.46% (n = 7, CSDS = 8 μmol dm). The throughput rate of the measurement is 4 samples per minutes. The proposed method can be applied to the determination of anionic surfactant in river, pond and house sewage water samples.